The present invention relates generally to the formation of thin epitaxial layers and relates more particularly to the formation of thin epitaxial layers by Chemical Vapor Deposition with dichlorosilane.
An integral part of the Bipolar Integrated Circuit is the epitaxial layer which serves as the collector and provides a highly ordered crystalline layer into which subsequent diffusions and implants of dopants can be made. A buried sub-collector is diffused or implanted into patterned areas of the substrate before the epitaxial layer is grown by, for example, Chemical Vapor Deposition (CVD). For fabrication of n-p-n transistors within the IC, n-type epitaxial silicon is deposited above the heavily doped n-type buried layer (sub-collector). The dopant concentration is typically about three orders of magnitude higher in the buried layer than in the epitaxial layer.
In view of the ever increasing desire to manufacture semiconductor devices with smaller geometries, the need has arisen to develop a satisfactory method for forming thinner epitaxial layers for bipolar devices. The conventional practice of depositing epitaxial layers at relatively high temperatures, for example in the temperature range of 1050.degree. C. to 1150.degree. C., does not produce epitaxial layers of optimum quality in the 1 to 2 micron range. Degradation of the epitaxy is primarily caused by updiffusion of the buried layer into the epitaxy due to the high temperature deposition. As a result, the usable, or effective, epitaxial thickness above the sub-collector is reduced by the extent of this updiffusion.
The obvious solution of lowering the epitaxial deposition temperature to minimize buried layer updiffusion creates a separate set of problems such as an increase in autodoping of n-type dopants and a deterioration of epitaxial layer quality.
From the foregoing it can be appreciated that a need exist for a low temperature process for forming thin epitaxial layers which does not result in degradation of the epitaxy.